The present invention relates to a control unit for an electric power steering apparatus that provides steering auxiliary force by motor to the steering system of an automobile or a vehicle. The invention particularly relates to a control unit for an electric power steering apparatus in which a desirable complementary sensitivity function is obtained, and a mechanical system and a control system are designed based on the complementary sensitivity function.
An electric power steering apparatus that applies auxiliary load to the steering apparatus of an automobile or a vehicle with turning effort of a motor applies the driving force of the motor to a steering shaft or a rack axis based on a transmission mechanism like gears or belts via a reduction gear. Such a conventional electric power steering apparatus carries out a feedback control of a motor current for accurately generating an assist torque (a steering auxiliary torque). The feedback control is for adjusting a motor application voltage so as to minimize a difference between a current control value and a motor current detection value. The motor application voltage is generally adjusted based on a duty ratio of a PWM (Pulse Width Modulation) control.
Approximately ten years have passed since the electric power steering apparatus appeared in the market, and the application of this apparatus has now been expanded to a vehicle class of 2,000 cc. At the same time, high-level performance of the steering apparatus has also been required. Recently, not only the performance of the electric power steering apparatus has reached the performance level of the conventional hydraulic power steering apparatus, new functions of the electric power steering apparatus have been developed, aiming at the advent of new value-added products.
A general structure of the electric power steering apparatus will be explained below with reference to FIG. 28. A shaft of a steering wheel is connected to a tie rod of running wheels through a torsion bar, a reduction gear, universal joints, etc. The shaft of the steering wheel is provided with a torque sensor for detecting a steering torque of the steering wheel. A motor for assisting the steering force of the steering wheel is connected to the shaft through a clutch (not shown) and the reduction gear. A control unit (ECU) for controlling the power steering apparatus is supplied with power from batteries through an ignition key (not shown). The control unit calculates a steering auxiliary command value I of an assist command based on a steering torque T detected by the torque sensor and a vehicle speed V detected by a vehicle speed sensor (not shown). The control unit then controls a current to be supplied to the motor based on the calculated steering auxiliary command value I. In FIG. 28, SAT represents a self-aligning torque.
The control unit is mainly composed of a CPU. FIG. 29 shows general functions to be executed based on a program inside the CPU.
Functions and operation of the control unit 30 will be explained below. A steering torque T detected by the torque sensor 10 and then input is phase-compensated by the phase compensator 31 for increasing the stability of the steering system. The phase-compensated steering torque TA is input to a steering auxiliary command value calculator 32. A vehicle speed V detected by the vehicle speed sensor 12 is also input to the steering auxiliary command value calculator 32. The steering auxiliary command value calculator 32 calculates a steering auxiliary command value I as a control target value of a current to be supplied to the motor 20, based on the input steering torque TA and the input vehicle speed V. The steering auxiliary command value I is input to a subtracter 30A, and is also input to a differential compensator 34 of a feedforward system for increasing a response speed. A difference (Ixe2x88x92i) calculated by the subtracter 30A is input to a proportional calculator 35, and is also input to an integration calculator 36 for improving the characteristic of a feedback system. Outputs from the differential compensator 34 and the integration calculator 36 are input to and added together by an adder 30B. A result of the addition by the adder 30B is obtained as a current control value E, and this is input to a motor driving circuit 37 as a motor driving signal. A motor current value i of the motor 20 is detected by a motor current detecting circuit 38, and this motor current value i is input to the subtracter 30A and is fed back.
FIG. 30 is a block diagram showing the transmission function of the electric power steering apparatus. In the drawing, xe2x80x9csxe2x80x9d represents a Laplace operator.
Particularly, the electric power steering apparatus has an advantage that it is possible to process information from the road surface and transfer the information to the driver to facilitate the driving, by utilizing the degree of freedom of designing the information. From FIG. 28 and FIG. 30, it can be understood that the sensitivity designing of the road information can be handled as a subject of defining a desirable transmission characteristic from the input of the road surface information to the delivery of the information to the steering torque.
In the mean time, the steering feeling desirable for the driver is realized by tuning the transmission characteristic from the steering angle to the steering torque. In general, the demand for clear steering wheel or quiet steering feeling depends greatly on this transmission characteristic. It can be understood from the schematic block diagram shown in FIG. 30 that these two specifications are in the trade off relationship. For example, this corresponds to a case where the driver feels friction in the steering as a result of the addition of friction to the power steering system in order to lower the sensitivity to the wheel flutter that occurs during a high-speed running of the vehicle.
In the field of the electric power steering apparatus, there has been demanded an advent of a product of which performance exceeds that of the hydraulic power steering apparatus while satisfying the performance of the electric power steering apparatus. Further, this product is desired to obtain stable and comfortable steering wheel, based on the designing of a control system and an electric control system that match the desirable complementary sensitivity function based on the road information.
The present invention has been made in the light of the above situations. It is an object of the present invention to provide a control unit for an electric power steering apparatus in which a desirable complementary sensitivity function is obtained, and a control system is designed to match this complementary sensitivity function.
The present invention provides a control unit for an electric power steering apparatus that controls a motor for giving steering auxiliary force to a steering mechanism based on a current control value calculated from a steering auxiliary command value calculated based on the steering torque generated in the steering shaft, and a current value of the motor. The object of the present invention can be achieved based on the arrangement that the complementary sensitivity function relative to a frequency is set to a level that approaches 1 in a band in which disturbance to be suppressed exists, and is set to a level that approaches 0 in a band in which disturbance to be transmitted exists.
Further, the object of the present invention can be achieved more effectively by the following arrangement. An eigenvalue of the power steering apparatus, an eigenvalue of suspension, and a flutter oscillation area and a motor torque ripple area are included in the band in which the disturbance to be suppressed exists. Alternatively, the eigenvalue of the power steering apparatus is set to 10 to 13 Hz, the eigenvalue of suspension is set to 13 to 17 Hz, the flutter oscillation area is set to 15 to 25 Hz, and the motor torque ripple area is set to 15 to 30 Hz. Alternatively, the complementary sensitivity function is obtained from a design of a mechanical control system and an electric control system. Alternatively, the mechanical control system is obtained from designs of a rolling-type rack and pinion mechanism, a rubber damper of a motor reduction gear mechanism, and a non-contact torque sensor.